This invention relates to an in-line type electron gun assembly utilized for an in-line multi-beam type colour picture tube and more particularly to an in-line type electron gun assembly including a plurality of grid structures in which corresponding grid electrodes of respective electron guns emanating electron beams are constructed as integral grid structures.
The in-line type colour picture tube of the class described above generally comprises a fluorescent surface 12 or screen emanating different colours (usually red, green and blue) and formed on the inner surface of the face plate 11 of a glass bulb 10 and a colour selection electrode 13 a predetermined distance spaced from the fluorescent surface. Further, in the neck portion 14 of the glass bulb is disposed an in-line type electron gun assembly 15 comprising three electron guns emanating a plurality (three) of electron beams toward the fluorescent surface 12. A deflection coil 16 and a convergence adjusting means 17 are mounted to surround the neck portion 14. The electron beams emanated from respective electron guns of the electron gun assembly are focused on one point of the colour selection electrode 13 by the action of the magnetic fields produced by the convergence adjusting means 17 and the deflecting coil 16 and then caused to impinge upon corresponding phosphor dots on the fluorescent surface.
The prior art electron gun assembly 15 of the in-line type has a construction as shown in FIG. 2 wherein the grid electrodes of respective electron guns having the same function are combined into a integral grid structure.
More particularly, as shown in FIG. 2, three cathode electrodes 22a, 22b and 22c are arranged side by side relationship along a line and are supported by a flat plate shaped cathode holder 21. Each cathode electrode is heated by a cathode heater 23 contained therein to emit electrons. In front of the cathode electrodes are disposed a first grid structure 24 for controlling the electron beams emanated by respective cathode electrodes, a second grid structure that accelerates the electron beams transmitting through the first grid structure, and third and fourth grid structures 26 and 27 in the form of short cylinders which are made of stainless steel, for example, and constitute an electron lens. These structures are secured to a glass bead 28 and function to cause the electron beams to impinge upon corresponding phosphor dots. The third grid structure 26 comprises upper and lower sections. Respective grid electrodes of the third and fourth grid structures 26 and 27 are also termed main lens electrodes and respectively comprise three juxtaposed circular openings 26a, 26b, 26c and 27a, 27b and 27c. The circular openings 27a, 27b and 27c provided for the fourth grid assembly 27 are made to have larger diameters than the circular openings 26a, 26b and 26c provided for the third grid assembly 26 for the purpose of focusing the electron beams transmitting through these openings on the fluorescent surface irrespective of the diameter and pitch of the circular openings, thus forming the main lens between the groups of the openings.
Auxiliary electrodes 29 and 30 in the form of cylinders are secured to respective openings for the purpose of preventing the side walls of the third and fourth grid structures 26 and 27 from affecting the electric fields formed therein.
To describe more in detail, the openings 26a, 26b, 26c and 27a, 27b, 27c are not disposed at equal distances from the side walls of the third and fourth grid structures but instead they are disposed at asymmetrical positions. Accordingly, when predetermined potentials are impressed upon these grid structures non-uniform or not concentric electric fields are formed near the openings thus forming a not point aberration which degrades the focusing characteristic. Such difficulty can be obviated by providing sleeves extending through respective grid structures from respective circular openings. Of course it is not necessary to provide such sleeves if integral sleeves or cylinders are formed to extend from the peripheries of respective openings. However, it is difficult to form three juxtaposed circular openings and integral sleeves having a desired length near the openings for flat plates which are used to form the third and fourth grid structures due to elongation of the flat plates. More particularly, the result of an experiment shows that when the length of the sleeves is made to be larger than 60% of the opening diameter it is possible to prevent the adverse effect of the side walls of respective grid structures 26 and 27 upon the electric fields near respective openings. To simultaneously form sleeves having the same length for respectively juxtaposed circular openings by a single drawing operation is extremely difficult, especially when the grid structures are made of stainless steel.